During the last decade, the field of scanning probe microscopy and scanning probe microscopes has grown. A scanning probe microscope (SPM) employs a sharp probe that is brought into proximity (often an atomic distance) to a sample surface, and scanned over a specified area of the sample surface. Through a variety of imaging mechanisms, the probe measures some property of the sample (most commonly topography) with very high spatial resolution. Many variations on the proximity probe theme have been engineered, allowing investigation of mechanical, electronic, magnetic, and optical properties of sample surfaces with spatial resolution in the atomic to few nanometer range in three dimensions.
In a typical use, the tip of the probe of an SPM is brought into close proximity (typically a few Angstroms) with the surface of a sample, until a sensing device detects a desired local interaction between the probe tip and the sample surface. The probe tip is scanned across the sample surface, while keeping the interaction strength between them constant by means of a feedback loop. Such scanning of the probe tip generates a contour map of constant probe—sample interaction strength. The contour map can typically be displayed on a monitor screen.
The most commonly utilized localized interactions between a probe tip and a surface being scanned are electron tunneling, van der Waals and atomic repulsive forces. SPMs that utilize electron tunneling include the scanning tunneling microscope, referred to as an “STM”. SPMs that utilize van der Waals and atomic repulsive forces include the atomic force microscope, referred to as an “AFM”. In both STM and AFM, the resulting contour map reveals detailed surface structure, in some cases with atomic scale resolution. This high resolution mandates mechanically stiff construction and high accuracy probe positioning capabilities which are typically achieved by a control computer and a positioner/scanner.
Other examples of SPMs include near-field scanning optical microscopes (NSOM), scanning tunneling optical microscopes (STOM), near-field scanning acoustical microscopes (NSAM), scanning capacitance microscopes (SCM), and scanning electrochemistry microscopes (SECM).
Researchers have discovered that an STM may be used to manipulate atomic structures. As discussed by D. M. Eigler & E. K. Schweizer in “Positioning single atoms with a scanning tunneling microscope,” Letters to Nature, Vol. 344, pp. 524–526 (Apr. 5, 1990), and U.S. Pat. No. 4,987,312 to Eigler, an STM may be utilized to position individual xenon atoms on a single-crystal nickel surface. By controlling tip-sample distances, it is possible to translate a xenon atom to a desired location via attractive forces between the STM tip and the xenon atom. Specifically, the translating process begins by locating a xenon atom deposited on the nickel surface by imaging the nickel surface in a non-perturbative imaging mode. The STM tip is then positioned directly above the xenon atom. The STM tip is lowered toward the atom by changing the tunneling current to a higher level, thus increasing the attractive interaction between the xenon atom and the STM tip. The STM tip is moved to the desired destination, thereby dragging the xenon atom with it. The STM tip is withdrawn by decreasing the tunneling current to the value used for imaging and the xenon atom remains placed approximately at the destination location. Corrugations in the surface potential of the nickel surface cause the xenon atoms to remain approximately at the desired destination position after removal of the STM tip.
Other researchers have explored the potential of using an STM to perform nano-fabrication. For example, H. Tang et al. describe positioning C60 molecules on a copper surface in “Fundamental considerations in the manipulations of a single C60 molecule on a surface with an STM,” Surface Science Vol. 386, pp. 115–123 (1997). In their article, the desired positioning of the C60 molecule is stabilized by an atomic step edge or a defect site on the copper surface. These researchers also analyzed several modes of manipulation, including a sliding mode, a pulling mode, and a pushing mode.
Other kinds of SPMs such as AFMs have been used to reposition and manipulate nanoscale objects. Baur et al. describe the manipulation of nanoparticles by means of repulsive forces exerted by the tip of an AFM on the nanoparticle to be manipulated. Baur et. al., Nanoparticle Manipulation by Mechanical Pushing: Underlying Phenomena and Real-Time Monitoring, Nanotechnology Vol 9, pp. 360–364 (1998).
Other SPM techniques have been developed to remove a nanoscale object from a surface and to place the object elsewhere on the surface instead of merely translating the particle over the surface. For example, one such SPM technique involves using an STM, and creating a sufficient attractive force between the STM tip and the nanoscale object to overcome binding forces between the object and the surface, and also to weakly bond the object to the STM tip. For example, application of a relatively high voltage to the STM tip may be utilized to induce an electric dipole in a given molecule to transfer the molecule to the STM tip. The molecule may then be repositioned to the desired location. The molecule is released by decreasing the voltage or by reversing the polarity of the voltage. See e.g., Huang et al., Deposition and Subsequent Removal of Single Si atoms on the Si(111)-7×7 Surface by a Scanning Tunneling Microscope, J. Vac. Sci. Technol. B 12(4), July/August 1994, pp. 2429–2433. Such STM techniques typically operate under extreme conditions (e.g., low temperature or high fields). Other SPM methods currently used for vertical manipulation (e.g., picking and placing) of molecules also require the use of voltage pulses, which generate very high fields.
In addition to the preceding techniques, Michelsen et al. have suggested that a molecule may be manipulated utilizing chemical driving forces between the molecule, the instrument manipulating the molecule, and the substrate involved. Michelsen et al., Assembler Construction by Proximal Probe, Fifth Foresight Conference on Molecular Nanotechnology, Nov. 5–8, 1997. Michelsen et al. suggested the transfer of silicon atoms from a gold “island” sputtered on a clean silicon (Si(100)-2×1) substrate to a clean area of the silicon surface where gold was not sputtered. The silicon atom is transferred from the gold island on the silicon surface to a clean area of the silicon surface by vertical manipulation with a tungsten SPM tip. Michelsen suggests that the chemical driving forces in the transfer of a silicon atom from the gold island to the tungsten tip, and then from the tungsten tip to the silicon substrate provide conditions where the silicon atom has less than 10 kcal/mol vibrational energy at each transfer point. However, Michelsen fails to recognize that when the silicon atoms are deposited onto the Si(100)-2×1 substrate having the gold islands sputtered thereon, the silicon atoms carry excess energy and therefore can easily migrate from inert areas, namely the gold islands, to highly reactive areas, namely the clean silicon substrate. The inert and reactive areas according to Michelsen are adjacent to each other during deposition of the atoms, thus, surface diffusion of the silicon atoms from the inert areas to the highly reactive sites is exacerbated. A significant number of molecules can selectively adsorb on these reactive areas, thereby precluding control over the placing of the molecules.
Although the preceding techniques related to manipulation of atoms and molecules represent significant technical accomplishments, the techniques still have substantial shortcomings. In particular, application of the techniques to nanoscale objects is extremely limited. Furthermore, many of the techniques are only operable under extreme conditions (e.g., very low temperatures or high fields), and many molecules cannot be repositioned with such techniques because the high electric fields, the high density tunneling current and other extreme conditions of such techniques would destroy the molecules. Further still, the stability of the atoms or molecules placed according to the preceding techniques is limited, as relatively minor forces may displace the particles in many cases. And finally, many nanoscale objects carry excess kinetic energy when they impinge on a surface during deposition, and therefore can easily migrate from the deposition site on the surface to a reactive area on the surface. The object will form a strong bond at the reactive area, thus permanently attaching the object to the surface at the wrong location. Therefore, it is not possible to create patterned surfaces with adjacent passive and reactive areas prior to the deposition process.
The present embodiments described below avoid these problems and are suitable for the fabrication of atomically precise patterns and structures.